Chemistry Reference
In-Depth Information
Even after Shell announced the closure of their plant for Carilon production
and despite the uncertain future of this commercial thermoplastic material, inves-
tigations are still being conducted. This is probably a consequence of the growing
interest in catalysis by late transition metals for the polymerization and co-poly-
merization of olefins. Comprehensive reviews of this subject have appeared [4, 5]
and will be taken as the basis for the following discussion.
8.2
Copolymerization Catalysts and Mechanism
The aforementioned commercialization of Carilon was achieved after catalytic sys-
tems, active enough to incorporate propene and ethene into the macromolecules,
were identified [3, 4]. The presence of small amounts of propene lowers the melt-
ing point of the polymer and enables processing of the material without degrada-
tion [6]. The catalytic systems used are complexes based on palladium of the type
[Pd(L-L
} is a chelate ligand, S is a solvent molecule,
and X is an anion with low coordination properties). These systems are usually
employed in the presence of an oxidant (e.g., a quinone) to avoid the formation of
inactive, reduced palladium species [7]. The catalytic species can be also formed
“
in situ
” from their components, namely a palladium salt (typically the acetate),
the ligand, and a strong acid that gives a non-coordinating anion. In general the
corresponding [Pd(L-L
)(S
2
)]X
2
(where L-L
{L=or L
)(S)(CH
3
CO)]X compounds were
used for mechanistic studies [8-11]. These species are considered to be useful
models for the two propagating species during the catalytic process. As typical li-
gands for the copolymerization reaction of ethene and aliphatic olefins, diphos-
phines of the type Ar
2
P(CH
2
)
n
PAr
2
were used, where the optimum value was
n
equal 3 and where Ar is equal to phenyl or
o
-methoxyphenyl [7, 12].
From the standpoint of the formation of the macromolecular chains, the identi-
fication of the end groups of the polymers is essential [7, 12-16]. Scheme 8.1
summarizes the identified chain end groups, as well as the reactions and the spe-
cies responsible for their formation. The relative concentration of these groups
changes according to the substrate and the nature of the catalytic system. Of the
commonly found groups such as saturated hydrocarbyl, alkoxycarbonyl, and vinyl,
only the latter can be assigned without alternative to terminating ends of the
chain. The
)(S)(CH
3
)]X or [Pd(L-L
-carbonylcarboxylate groups, which can probably also be assigned
only to chain terminations, were identified for low molecular weight carbonylation
products when the copolymerization reactions were carried out in the presence of
a large excess of the oxidant [17]. Furthermore, labeling experiments using
CH
3
OD suggest that termination by methanolysis may involve an enolate-inter-
mediate, which is formed by isomerization of the alkyl
intermediate that
is
formed after ethene insertion [18].
Scheme 8.2 presents the mechanistic description of the formation of the macro-
molecules. Various pathways have been proposed to account for the first forma-
tion of the hydride-initiating species [4, 19].
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